modifoo » t-stop http://modifoo.com film | technology | life Thu, 20 Nov 2008 13:52:29 +0000 http://wordpress.org/?v=2.6.3 en What’s a T-Stop, anyway? http://modifoo.com/2007/11/24/whats-a-t-stop-anyway.html http://modifoo.com/2007/11/24/whats-a-t-stop-anyway.html#comments Sat, 24 Nov 2007 11:17:40 +0000 Martin http://modifoo.com/2007/11/24/whats-a-t-stop-anyway.html Anyone familiar with photography knows about f-stops. Calculated as Focal Length divided by Diameter (of the front lens element). The smaller the value of the f-stop, the more light passes the lens, the shorter exposure time necessary, the narrower the depth of field, the lower ASA/ISO rating necessary.

Typical f-stop values are f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 and f/32, but there are lenses with f-stops as low as 1 and as high as 128. With the arrival of automatic exoposure meters, fractional f-stops have also become the norm.

T-Stop

When you start using professional film lenses, you will come across the phrase T-stop. While the f-stop is a mathematically derived value, the T-stop is a calibrated value. Since lenses have optical elements, and each of these elements blocks a (tiny) amount of light, the actual amount of light coming through a lens is always lower than the f-stop value suggests.

Therefore most cine lenses are individually calibrated to give the actual amount of light transmitted, the T-stop (T standing for Transmission.) T-stops are always (slightly) higher than f-stops.

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